Electric instruments are used in the field of electronics, and automobile applications, in various forms such as a primary battery, a secondary battery, and a capacitor. Particularly, a lithium ion battery, which is a type of non-aqueous electrolyte battery, is widely used as a power supply of mobile instruments such as a mobile phone and a notebook-type personal computer because of its properties such as its high energy density. Secondary batteries capable of being recharged, are increasing in importance due to consideration for environmental problems, and thus research has been conducted into the application of such secondary batteries to automobiles, electric chairs, and home/business power storage systems, in addition to the mobile instruments.
When producing an electric instrument such as a lithium battery, it is required to use an adhesive having excellent insulating properties and heat resistance taking electrical short circuit or thermal ignition into consideration.
JP 2011-052049 A discloses that a hot-melt adhesive including a copolymer of ethylene with a carboxylic acid derivative having an ethylenic double bond, a polypropylene wax, a polyethylene wax, and a Fischer-Tropsch wax is suited for electric instruments (see JP 2011-052049 A, Claim 1 and Examples). A trial is made on an improvement in insulating properties of the electric instruments by coating an electrode and a separator with the hot-melt adhesive of the same document (see JP 2011-052049 A, [0055] and [0069] to [0072]).
An examination has recently been made on the use of a light-weight and thin laminated film, which is obtained by laminating a metal foil such as an aluminum foil on a pair of insulating resin films thereby integrating them, as an external container, so as to improve the energy density of this kind of battery. An external container obtained by forming a laminated film into a bag or tray shape is used as a container for lithium secondary batteries such as a lithium ion battery and a lithium polymer battery, and a capacitor.
The lithium ion battery is produced by laminating a positive electrode material, a negative electrode material, and a separator to each other; inserting the obtained laminate into an external container; filling the external container with an electrolytic solution; inserting a positive electrode lead and a negative electrode lead; and sealing the container.
The lithium polymer battery is produced by inserting a battery body into an external container, which battery body is obtained by laminating a positive electrode material, a negative electrode material, and a polymer electrolyte layer permeated with an electrolytic solution; inserting a positive electrode lead and a negative electrode lead so that one end is located at each electrode material and the other end is located outside the container; and finally sealing the container.
The capacitor is produced by inserting a laminate into an external container, which laminate is obtained by interposing an electrode of activated carbon permeated with an electrolytic solution and a separator between current collectors of aluminum; and sealing the container in a state where one end of the current collector is extruded out of the container.
As shown in FIG. 1, an external container is produced by forming an adherend 30 made of a resin film such as a polyimide film into a bag shape, and the container is filled with an electrolytic solution 20. The adherend 30 is sealed with an adhesive A so that the electrolytic solution 20 does not leak from the container.
Various examples have been reported on external containers of batteries.
JP 2011-060501 A discloses a packing material for a lithium ion battery, that is, an external container. The same document discloses, as an adhesive resin layer composing the packing material, a mixed resin of a maleic anhydride modified polypropylene resin and a maleic anhydride modified styrene based elastomer resin (see JP 2011-060501 A, [0041]). The document also teaches that the maleic anhydride modified polypropylene resin, having a specific Vicat softening point, is likely to prevent a decrease in adhesion force in the event of using a hot-melt type adhesive (see JP 2011-060501 A, [0025]). It is necessary for the packing material of the same document to include a lot of layers such as a base material layer, an adhesive layer, an aluminum foil layer, a chemically treated layer, and a sealant layer in addition to the adhesive resin layer (see JP 2011-060501 A, FIG. 1).
The packing material for a lithium ion battery of such multilayered films contains an electrolytic solution, and is therefore sealed so as not to leak the material contained therein (see JP 2011-060501 A, [0004]). One of sealing means includes a method in which the end of the packing material 30 is heat-sealed with the hot meltadhesive A, as shown in FIG. 1. However, an adhesive having no resistance to the electrolytic solution 20 is likely to cause swelling, and thus fails to obtain sufficient adhesion.
JP 8-106884 A discloses a film for sealing material of a thin battery, and also discloses that a modified polyolefin is used as a material of a hot-melt adhesive polyolefin layer (see JP 8-106884 A, [0015] and [0028]). As is apparent from Examples of the same document, electrolytic solution resistance is improved to some extent (see JP 8-106884 A, [0062] and [0065]), but it is difficult to say that the electrolytic solution resistance sufficiently meets the high-level requirements of users in recent days.
Furthermore, no study has been made on adhesion with a polyimide film having excellent electrolytic solution resistance (solvent resistance) which has been used more often than before in recent years. From this point of view, it is impossible to say that the adhesives of JP 2011-052049 A, JP 2011-060501 A, and JP 8-106884 A have sufficient performances.